Barry Scwhartz posted this week about a Google query hack, which may or may not enable you to confirm that your site was hit by Google's Penguin update. Schwartz credits WebmasterWorld member Martin Ice Web with the find.
The user added a negative parameter for amazon with his keywords, and Google returned his site to pre-Penguin rankings. "For example, he search for [blue widget -amazon] and his rankings pre-Penguin showed up on Google for the query [blue widget]," Schwartz explains.
It didn't seem very reliable at first, so I didn't bother to cover it. Even Schwartz noted that it failed on some of the queries he tried. However, the comments on his article have poured in since then, and it sounds like it has worked for quite a few people, so perhaps it's worth taking a look at.
One commenter notes that other negative keywords seem to work as well, such as "-ebay".
Remember, if you believe that you were unfairly hit by the Penguin update, Google has a form where you can complain. Of course, it's likely that Google disagrees. After all, they consider the update to be a success.